cmd/compile: consistently use s.pp in genssa

cmd/compile: consistently use s.pp in genssa

In genssa, s.pp == pp, so using either is equivalent, but use is
inconsistent. About half of the uses use s.pp and the other half use pp.

This gets confusing, especially when two different uses are right next
to each other, because it implies that these might be different.

Pick one and use it consistently.

Change-Id: Ifb1bb9332138d8cb62a45c212fcd7139f8511901
Reviewed-on: https://go-review.googlesource.com/c/go/+/560780
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>